1. Field of the Invention
The present invention relates to a valve-integrated transponder that is integrated with an air valve attached to a wheel rim and is disposed in a tire, and that is used in a system for monitoring the air pressure of the tire.
2. Description of the Related Art
In recent years, more and more vehicles are equipped with a system that enables a driver at the driver seat to monitor the air pressure of the tires. Specifically, such a system is provided for allowing a driver to quickly check the air pressure of the tires and to see whether the air pressure is abnormal while driving a vehicle, and is realized by installing transponders equipped with detecting elements and antenna elements in appropriate positions within the tires. A transponder of this type is integrally joined to, for example, an end of an air valve that is located within a tire, so that when the air valve is attached to a wheel rim, the valve-integrated transponder becomes disposed facing a well of the wheel rim. See Japanese Unexamined Patent Application Publication No. 2006-69389 (pages 3 to 5, and FIG. 1) for an example of such a valve-integrated transponder. Although the type of built-in antenna to be used in a transponder contained inside a tire is not particularly limited, an inverse F antenna element made by forming a metal plate is advantageous in terms of low fabrication cost and compactness.
A valve-integrated transponder of the related art normally contains a battery power source. However, with a structure equipped with a battery, the frequency of detecting the air pressure of a tire will need to be minimized to expand the life of the battery, and a complicated process will inevitably be necessary when replacing the battery. Recently, a valve-integrated transponder that does not require a battery power source has been proposed. Specifically, in this valve-integrated transponder, the antenna element inside the tire is excited by a radio wave sent from an external antenna in the vehicle body and is supplied with a signal current based on information detected by the detecting element. Such a valve-integrated transponder not requiring a battery can allow for an increase in the frequency of detecting the air pressure of the tire and can thus achieve higher detection accuracy. In addition, the valve-integrated transponder also allows for lower maintenance costs due to not requiring a battery replacement process.
Generally, a radio wave radiating from an antenna element of a valve-integrated transponder is transmitted to an external antenna in the vehicle body by passing through a side wall of a tire. If the radio wave used has a short wavelength and has a strong property of rectilinear propagation, such as a 2.4 GHz radio wave used in a transponder that does not require a battery, it is necessary to increase radiation components of a direct wave directed from the antenna element in the tire towards the side wall. However, with a common inverse F antenna element, a radio wave radiates both upward and sideward from a radiation conductor. This makes it difficult for the radio wave to radiate efficiently towards the side wall of the tire. If a common inverse F antenna element is contained in a valve-integrated transponder, and the antenna element is oriented such that the side wall is located next to the radiation conductor of the antenna element, the radiation conductor will be positioned near a wall surface of the wheel rim where a valve hole is located. This tends to cause an electric field to concentrate in a space between the radiation conductor and the wheel rim, which can cause the radiation components of a direct wave directed towards the side wall to become reduced.